Gas chromatographs (GC's) employ the use of open tubular capillary columns in order to affect a separation of chemical constituents contained in a sample mixture. The history of the development of modern capillary columns can be found in an article by S. R. Lipsky entitled “The Fused Silica Glass Capillary Column for Gas Chromatography—The Anatomy of a Revolution” Journal of Chromatography Library, Volume 32, 1985, pages 257-282. Since this time, the “anatomy” of GC columns has remained largely unchanged.
Accuracy, and more importantly, precision of the flow rate through a capillary column is necessary for repeatable retention times, critical to component identification. The small bore and high pressure drop of these capillary columns result in small volumetric flow rates, which are difficult to precisely control when closing the control loop using a flow sensing element. Since this is the case, a calculated flow is generally used instead by closing the control loop using a pressure sensing element. This is commonly referred to as “Electronic Pressure Control” or EPC. The relationship of flow to pressure is given by the well known Poiseuille equation using prior knowledge of the gas viscosity, column dimensions, inlet and outlet pressures.
                                          ⅆ            V                                ⅆ            T                          =                                            π              ⁢                                                          ⁢                              r                4                                                    16              ⁢                                                          ⁢              η              ⁢                                                          ⁢              L                                ⁢                      (                                          (                                                      p                    i                    2                                    -                                      p                    o                    2                                                  )                                            p                o                                      )                                              Equation        ⁢                                  ⁢        1            where:
Pi inlet pressure
Po outlet pressure
L is the length of the column
η is the viscosity of the gas
r is the column internal radius
As can be seen in Equation 1 above, the column radius term is raised to the fourth power. It is thus desirable that the actual radius is known to a high degree of accuracy in order to avoid multiplicative errors. This is especially important to achieve the highest analytical reproducibility column to column and instrument to instrument. The stated internal diameter (2× radius) as reported from a column manufacturer is a “nominal” one and subject to inaccuracy.
Methods are known in the art for more accurately determining the column radius (and thus the diameter) by measuring column flow rate accurately in accordance with Equation 1, using a high enough pressure drop which results in a large enough flow to accurately be measured. Once the radius is known, lower pressures can then be used to set the column flow through calculation also in relation to Equation 1.
Another known technique for measuring the column internal diameter involves measuring the retention time of an “un-retained” peak to determine average linear velocity. This technique does not require an accurate flowmeter, as time can be measured accurately. Regardless of which of these techniques is used for determining column ID, it is necessary to know in advance the accurate column length in order to make the determination.
Capillary column lengths are subject to an initial error from the manufacturer as well as operator induced uncertainties. As a matter of routine practice, columns are trimmed in length in order to re-establish adequate performance. The column end inserted into a detector and more particularly the end inserted into a chromatograph inlet need to be trimmed when fouled with non-volatile sample matrix, particles from septa or ferrules, oxidation from excessive heat and oxygen exposure. Trimming GC columns in this manner necessitates the keeping of a log book which notes the total remaining column length so that proper values can be entered into the EPC control algorithm. In addition, the logbook must associate this information with each particular column so that columns may be disposed of when they reach a lower practical length. Even if a logbook is maintained, the error in cut lengths can stack up, eventually resulting in a column with an inaccurate known length. Measuring the column length by the use of a ruler is impractical for all but the shortest GC columns, as often columns are in excess of 15 meters so that adequate separation can be accomplished. It is also impractical to unwind these columns from their supplied cages as doing so can damage the fragile column.